Evolving force balance during incipient subduction

Nearly half of all active subduction zones initiated during the Cenozoic. All subduction zones associated with active back arc extension have initiated since the Eocene, hinting that back arc extension may be intimately associated with an interval (several tens of Myr) following subduction initiation. That such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates. Subduction initiation is known to have occurred in a variety of tectonic settings: old fracture zones, transform faults, and extinct spreading centers and through polarity reversal behind active subduction zones. Although occurring within different tectonic settings, four known subduction initiation events (Izu-Bonin-Mariana (IBM) along a fracture zone, Tonga-Kermadec along an extinct subduction boundary, New Hebrides within a back arc, and Puysegur-Fiordland along a spreading center) were typified by rapid uplift within the forearc followed by sudden subsidence. Other constraints corroborate the compressive nature of IBM and Tonga-Kermadec during initiation. Using an explicit finite element method within a two-dimensional domain, we explore the evolving force balance during initiation in which elastic flexure, viscous flow, plastic failure, and heat transport are all considered. In order to tie theory with observation, known tectonic settings of subduction initiation are used as initial and boundary conditions. We systematically explore incipient compression of a homogeneous plate, a former spreading center, and a fracture zone. The force balance is typified by a rapid growth in resisting force as the plate begins bending, reaching a maximum value dependent on plate thickness, but typically ranging from 2 to 3 × 1012 N/m for cases that become self-sustaining. This is followed by a drop in stress once a shear zone extends through the plate. The formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall. Cumulative convergence, not the rate of convergence, is the dominant control on the force balance. Viscous tractions influence the force balance only if the viscosity of the asthenosphere is >1020 Pa s, and then only after plate failure. Following plate failure, buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction. The total work done is also influenced by the rate of lithospheric failure. A self-sustaining subduction zone does not form from a homogeneous plate. A ridge placed under compression localizes subduction initiation, but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate. The large initial bending resistance can be entirely eliminated in ridge models, explaining the propensity for new subduction zones to form through polarity reversals. A fracture zone (FZ) placed in compression leads to subduction initiation with rapid extension of the overriding plate. A FZ must be underthrust by the older plate for ~100–150 km before a transition from forced to self-sustaining states is reached. In FZ models the change in force during transition is reflected by a shift from forearc uplift to subsidence. Subduction initiation is followed by trench retreat and back arc extension. Moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of Pacific subduction zones. The models provide an explanation for the compressive state of western Pacific margins before and during subduction initiation, including IBM and Tonga-Kermadec in the Eocene, and the association of active back arcs with young subduction zones. On the basis of our dynamic models and the relative poles of rotation between Pacific and Australia during the Eocene, we predict that the northern segment of the Tonga-Kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self-sustaining states

Letter to James Murphy from LaVier L. Staven

A letter to Dr. James Murphy regarding funding for the Military Science programhttps://scholars.fhsu.edu/military_science_rotc_docs/1013/thumbnail.jp

The role of inheritance in structuring hyperextended rift systems: Some considerations based on observations and numerical modeling

International audienceA long-standing question in Earth Sciences is related to the importance of inheritance in controlling tectonic processes. In contrast to physical processes that are generally applicable, assessing the role of inheritance suffers from two major problems: firstly, it is difficult to appraise without having insights into the history of a geological system; and secondly all inherited features are not reactivated during subsequent deformation phases. Therefore, the aim of this paper is to give some conceptual framework about how inheritance may control the architecture and evolution of hyperextended rift systems.In this paper, we use the term inheritance to refer to the difference between an “ideal” layer-cake type lithosphere and a “real” lithosphere containing heterogeneities. The underlying philosophy of this work is that the evolution of hyperextended rift systems reflects the interplay between their inheritance (innate/“genetic code”) and the physical processes at play (acquired/external factors). Thus, by observing the architecture and evolution of hyperextended rift systems and integrating the physical processes, one may get hints on what may have been the original inheritance of a system.We first define 3 types of inheritance, namely structural, compositional and thermal inheritance and develop a simple and robust terminology able to describe and link observations made at different scales using geological, geophysical and modeling approaches. To this, we add a definition of rift-induced processes, i.e. processes leading to compositional changes during rifting (e.g. serpentinization or decompression melting). Using this approach, we focus on 3 well-studied rift systems that are the Alpine Tethys, Pyrenean–Bay of Biscay and Iberia–Newfoundland rift systems. However, as all these examples are magma-poor, hyperextended rift systems that evolved over a Variscan lithosphere the concepts developed in this paper cannot be applied universally. For the studied examples we can show that:1) the inherited structures did not significantly control the location of breakup2) the inherited thermal state may control the mode and architecture of rift systems, in particular the architecture of the necking zone3) the architecture of the necking zone may be influenced by the distribution and importance of ductile layers during decoupled deformation and is consequently controlled by the thermal structure and/or the inherited composition of the crust4) conversely, the deformation in hyperextended domains is strongly controlled by weak hydrated minerals (e.g. clay, serpentinite) that result from the breakdown of feldspar and olivine due to fluid and reaction assisted deformation5) inherited structures, in particular weaknesses, are important in controlling strain localization on a local scale and during early stages of rifting6) inherited mantle composition and rift-related mantle processes may control the rheology of the mantle, the magmatic budget, the thermal structure and the localization of final rifting.These key observations show that both inheritance and rift-induced processes played a significant role in the development of the magma-poor southern North Atlantic and Alpine Tethys rift systems and that the role of inheritance may change as the physical conditions vary during the evolving rifting and as rift-induced processes (serpentinization; magma) become more important. Thus, it is not only important to determine the “genetic code” of a rift system, but also to understand how it interacts and evolves during rifting. Understanding how far these new ideas and concepts derived from the well-studied hyperextended rift systems of the southern North Atlantic and Alpine Tethys can be translated to other less explored hyperextended rift systems will be one of the challenges of future research in rifted margins

Correction to "Extreme crustal thinning in the Bay of Biscay and the Western Pyrenees: From observations to modeling"

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Jeu vidéo et mort(s) : analyse de l’imaginaire de la mort dans le jeu vidéo

Dans notre société où la mort est taboue, où le deuil doit être vécu en cachette et le sujet évité dans les sphères publiques, deux sphères ont cependant le loisir de l’aborder de manière attendue : la religion et l’art. L’art qui a retenu mon attention est le jeu vidéo pour sa capacité à mettre le joueur au contrôle de sa mort. Chaque œuvre vidéoludique présente un univers différent et chacune traitera différemment le joueur lorsqu’il échoue à une tâche. En m’intéressant à ces punitions et aux morts numériques qui en découlent, je traite de l’imaginaire mythique auquel est confronté le joueur en jouant à des jeux vidéo. Ce mémoire se découpe en quatre chapitres. Le premier chapitre pose les thématiques et les concepts opératoires de la recherche. Une première section porte sur le jeu, puis plus spécifiquement sur le jeu vidéo. Une fois les définitions posées, je me suis intéressé à la possibilité du jeu à influencer le joueur. Dans une deuxième section, j’aborde la mort ainsi que les discours sur celle-ci (eschatologiques) et je commence à l’associer avec le jeu vidéo. Dans le deuxième chapitre, j’aborde la problématique, à savoir que si la plupart des arts présentent une mort définitive, le joueur ne peut mourir dans le jeu vidéo. Cette problématique s’articule autour de la notion de l’évolution et de l’atteinte de l’immortalité. Le troisième chapitre présente mon cadre théorique et ma méthodologie. J’explique la formation de mon corpus de jeux vidéo et j’explique comment il a été analysé. Le jeu vidéo est appréhendé comme un récit mythique capable de nous informer sur les mystères de la mort. Dans ce contexte, j’utilise la mythanalyse développée par Gilbert Durand pour déterminer dans chacun des jeux le rapport entre le joueur et la mort. Le dernier chapitre présente mes résultats en regroupant les jeux par types. Je présente le rapport entre le joueur et la mort dans chacun des types ainsi que dans les jeux présentant un rapport unique à la mort.In our society where death is a taboo, where mourning must be lived in secret and the topic avoided in public spheres, two spheres have the leisure to approach it without difficulty: religion and art. The art that caught my attention is the video game for its ability to put the player in control of his death. Each video game presents a different universe, and each will treat the player differently when he fails a task. By taking an interest in these punishments and the digital deaths that ensue, I examine the mythical imaginary that the gamer faces while playing video games. This thesis is divided into four chapters. The first chapter sets out the themes and concepts of my research. The first section deals with games, then more specifically with video games. I then address the ways by which the game can influence the player. In a second section, I discuss death and the discourses on it (eschatological) and I begin to associate it with video games. In the second chapter, I develop my problematic, namely that if most of the arts present a final death, the player cannot truly die in video games. This issue is woven around the notion of evolution and the attainment of immortality. The third chapter presents my theoretical framework and my methodology. I explain the formation of my video game corpus and explain how it was analyzed. The video game is understood as a mythical story capable of informing us about the mysteries of death. In this context, I use the mythanalysis developed by Gilbert Durand to determine in each game the relationship between the player and death. The last chapter presents my results by grouping the games by type. I present the relationship between player and death in each of the types, as well as in the games with a unique relationship to death

Assessing the impact of orogenic inheritance on the architecture, timing and magmatic budget of the North Atlantic rift system: a mapping approach: Orogenic inheritance and the North Atlantic rift

International audienceIn order to investigate the impact of orogenic inheritance on the characteristics of hyperextended rift systems we develop new mapping methods highlighting the first-order architecture and timing of hyperextended rifts, as well as the distribution of heterogeneities inherited from previous orogenies. We use these to characterize the North Atlantic rift system and adjacent areas affected by the Palaeozoic Caledonian and Variscan orogenies. Comparison of these maps demonstrates major differences in the behaviour of the North Atlantic rift relative to both orogens, the Variscan front appearing to be a major limit. Indeed, the rift cuts through the Caledonian orogen and parallels its structural grain, while it circumvents the core of the Variscides. In addition, rifting is protracted and polyphase, and breakup is magma-rich North to the Variscan front, as opposed to the South where a single, apparently continuous extensional event lead to magma-poor breakup in less than 50 Myr. These observations point to a major influence of orogenic inheritance on the characteristics of hyperextended rift systems. On the other hand, our study supports that rifts reactivate sutures corresponding to former large (> 2 000 km) oceans, while leaving sutures of small (< 500 – 1 000 km) oceanic basins little affected, suggesting a significant impact of the pre-orogenic histories on subsequent extensional processes

Physical Performance and Skeletal Muscle Transcriptional Adaptations Are Not Impacted by Exercise Training Frequency in Mice with Lower Extremity Peripheral Artery Disease.

Exercise training is an important therapeutic strategy for lower extremity peripheral artery disease (PAD). However, the effects of different exercise frequency on physiological adaptations remain unknown. Thus, this study compared the effects of a 7-week moderate-intensity aerobic training performed either three or five times/week on skeletal muscle gene expression and physical performance in mice with PAD. Hypercholesterolemic male ApoE-deficient mice were subjected to unilateral iliac artery ligation and randomly assigned to sedentary or exercise training regimens either three or five times/week. Physical performance was assessed using a treadmill test to exhaustion. Expression of genes related to glucose and lipid metabolism, mitochondrial biogenesis, muscle fiber-type, angiogenesis, and inflammation was analyzed in non-ischemic and ischemic gastrocnemius muscles by real-time polymerase chain reaction. Physical performance was improved to the same extent in both exercise groups. For gene expression patterns, no statistical differences were observed between three or five times/week exercised mice, both in the non-ischemic and ischemic muscles. Our data show that exercising three to five times a week induces similar beneficial effects on performance. Those results are associated with muscular adaptations that remain identical between the two frequencies

Modes of faulting at mid-ocean ridges

Abyssal-hill-bounding faults that pervade the oceanic crust are the most common tectonic feature on the surface of the Earth. The recognition that these faults form at plate spreading centres came with the plate tectonic revolution. Recent observations reveal a large range of fault sizes and orientations; numerical models of plate separation, dyke intrusion and faulting require at least two distinct mechanisms of fault formation at ridges to explain these observations. Plate unbending with distance from the top of an axial high reproduces the observed dip directions and offsets of faults formed at fast-spreading centres. Conversely, plate stretching, with differing amounts of constant-rate magmatic dyke intrusion, can explain the great variety of fault offset seen at slow-spreading ridges. Very-large-offset normal faults only form when about half the plate separation at a ridge is accommodated by dyke intrusion